This invention relates to a negative electrode material for lithium-ion batteries comprising silicon and having a chemically treated or coated surface influencing the zeta potential of the surface.
The Li-ion technology has dominated the portable battery market and stands as a serious candidate for EVs and HEVs applications. Therefore for such large volume applications, besides the need for higher energy density and power rate electrodes, safety and cost issues must be overcome. To address this challenge, a wide variety of research directions enlisting different Li reactivity mechanisms (conversion, displacement and alloying reactions) as compared to the classical Li insertion mechanism have been explored with more or less success. Recent findings have definitively put nano-materials on the stage with fast implementations in commercial cells, both at the positive side with intercalation compounds (olivine LiFePO4) and at the anode side with alloying reactions.
Experimental studies on the electrochemical alloying of elements with lithium started in the early 70's when Dey described the creation of Li alloys at room temperature with Sn, Pb, Al, Au . . . and pointed out they were similar to those prepared by metallurgical ways. In 1976, Li was found to electrochemically react with Si at high temperatures through the consecutive formation of Li12Si7, Li14Si6, Li13Si4 and Li22Si5 phases. Even if we are now aware that this electrochemical reaction is limited at room temperature to the Li15Si4 end-member both the corresponding gravimetric
(3579 mAh/g) and volumetric (8330 mAh/cm3) capacities are far ahead all the other Li-uptake reactions so far identified, regardless of their nature (intercalation, alloying, conversion), and self-justify the past, present and surely future focus on this system.
Therefore, an inherent drawback to electrodes based on alloys lies in their poor cycling life (e.g. rapid capacity fading) caused by the large volume swings upon subsequent charges/discharges, which results in an electrochemical grinding of the electrode, and hence its electric percolation loss, and in the mean time a huge electrolyte degradation on the particles surface. To avoid this issue the first optional solution was to use electrodes made of nanoparticles as the usual mechanisms of deformation and dislocation are not the same as the micro scale, with namely the small particles being capable of releasing strains without fracturing. Unfortunately, these nanoscale silicon powders rapidly oxidize when exposed to air and the surface of commercially available Si or Si made in-situ in a plasma process, as disclosed in WO2012-000858, is covered by (protonated) silanol groups SiOH. These surface silanol groups cause a non-optimal behavior of the silicon particles in the anode electrode during the life of the battery.
Other approaches to keep the electrode integrity are either
1) the preparation of metal thin-films, which provide the best electrical contact via their strong adherence to the substrate, hence enabling high quality electrical contact during cycling, or 2) the fabrication of (metal-carbon-binder) composite electrodes with the proper binder so that Si/C/Carboxy-methyl-cellulose (CMC) electrodes having attractive cycling properties are achieved, or 3) the elaboration via the pyrolysis of organic precursors of metal-carbon (Si/C, Sn/C) composites with the carbon acting as a volume buffer matrix.
Huge progress has been made since 10 years to improve the behaviour of the silicon based electrode by an improvement of the slurry preparation and the choice of a good binder. Nevertheless, the active material particles need to be further tuned to realize the next step of improvement, especially to provide a better capacity retention during cycling. If today, the anode capacity can be maintained with a lithium counter-electrode, the silicon based metal-based electrodes present generally a weak capacity retention with a cathode that contains a limited quantity of lithium. To avoid this issue, the silicon surface needs to be modified to limit the electrolyte degradation.
Different strategies can be used to modify and protect the silicon surface. But the new surface needs also to be chosen according to the reactivity of the electrolyte, since the material needs to increase the potential window of stability of the electrolyte, and hence decrease the electrolyte decomposition. In US2011/0292570 Si nanoparticles having a positive surface charge are coating with graphene. The positive charge is caused by modification of the surface of the nanoparticles by functional groups, which are preferably selected from amino groups and ammonium groups, such as NR2 and NR3+, where R is selected from H, C1-C6-alkyl or -hydroxyalkyl.
The invention aims at disclosing new Si based particles used in the negative electrode of a rechargeable battery that are capable of providing a better capacity retention during cycling.